Backpack



FIG. 1 is a front perspective view of a backpack;

FIG. 2 is a front view of a backpack;

FIG. 3 is a rear view of a backpack;

FIG. 4 is a left side view of a backpack;

FIG. 5 is a right side view of a backpack;

FIG. 6 is a top view of a backpack; and,

FIG. 7 is a bottom view of a backpack.

The broken lines illustrate portions of the backpack and form no part of the claimed design. 

CLAIM The ornamental design for a backpack, as shown and described. 